Lens antenna



33 w T? M Jan. 3, 1956 c. F. CRAWFORD 2,739,316

LENS ANTENNA Filed Aug. 30, 1954 I N V EN TOR.

United States Patent LENS ANTENNA Carl F. Crawford, Camden, N. 3., assignor to Radio Corporation of America, a corporation of Delaware Application August 30, 1954, Serial No. 452,804 8 Claims. 01. 343-753 The present invention relates to a lens antenna and in particular to an improved mounting arrangement for securing a lens shield to the lens focusing elements.

A dielectric lens shield is often employed to reduce the effects of weather and wind loading on a lens antenna. conventionally the shield is mounted to the peripheral edge or rim of the lens. This has been found to be unsatisfactory in applications wherein the matching of the feed to the lens must be maintained constant during operation.

The lens shield, although substantially transparent to radio waves, does reflect back to the feed a small portion of the energy transmitted thereto. This energy is ordinarily balanced out by means located in the feed system, to obtain a proper match between the feed and the lens and, when employing a multielernent feed system, to obtain the proper degree of isolation between feed elements. When conventionally mounted, moderate amounts of wind may bend the shield and change the spacing between the center portion of the shield and the lens. This substantially changes the amount of balanced and unbalanced reflection to the feed thereby deteriorating the match and isolation beyond allowable limits.

It is an object of the present invention to provide a radio wave lens system in which the above defect is minimized.

It is a further object of the present invention to provide an improved shield mounting for a lens shield which prevents the shield from bending toward or away from the lens and which thereby prevents changes in the amount of energy reflected back from the lens shield to the feed element.

It is yet another object of the present invention to provide an improved mounting for a lens shield which is much sturdier than prior mountings and which has substantially no effect on the lens radiation pattern.

A typical embodiment of the present invention comprises a lens for focusing radio waves into a beam pattern having null portions adjacent the lens and a shield substantially transparent to radio Waves mounted to the lens in the path of the beam pattern by means of supports or mounting elements located in the null portions of the beam pattern. When so arranged, the supports have substantially no effect on the beam pattern.

In a preferred embodiment of the invention the lens consists of a circularly stepped lens illuminated by a feed located at the focus of the lens. The result is that the beam pattern includes concentric null or shadow portions. The lens shield is mounted to the edges of the lens focusing elements (or slats) by supports located in the null or shadow portions of the beam pattern. As many supports may be used as required to obtain the necessary lens shield rigidity according to the maximum Wind load expected to be encountered.

The invention will be described in greater detail by reference to the following description taken in connection with the accompanying drawing in which:

Fig. 1 is a front view of a typical radio wave lens according to the present invention;

Fig. 2 is a cross-sectional view along line 2-2 of Fig. 1;

Fig. 3 is an enlarged view, partially in cross-section, of one embodiment of a lens mounting structure in accordance with the present invention;

Fig. 4 is a cross-sectional view along line 4-4 of Fig. 3; and

Fig. 5 is an enlarged cross-sectional view of another embodiment of a mounting structure according to the present invention.

In the figures similar reference numerals refer to similar elements.

Referring now to Figs. 1 and 2, the lens comprises a plurality of stepped metallic slats 11, one of which is shown in side view in Fig. 2. The lens is illuminated by a radiating element such as horn feed 12 or any other type of conventional feed element such as one or more dipoles, a pillbox, or the like. In an embodiment of the invention actually constructed the feed element comprised a so-called four horn feed which was used for purposes of simultaneous lobe comparison tracking. Mounted to the lens It! is a dielectric lens shield 13 which is substantially transparent to radio waves but which reflects back a small portion of the energy transmitted thereto by feed element 12. This reflected energy is normally balanced out by means, not shown, in the feed element system so that the required match is obtained between the feed element and lens and so that suflicient isolation is obtained betwen elements in a multielement feed.

In prior art systems of the type described, the lens shield is mounted to the lens by means of supports located about the rim 14 0f the lens. This type of mounting has two serious disadvantages. One is that the support or supports cause some degree of distortion of the lens pattern. The second and perhaps more important disadvantage is that the entire center area of the lens shield is unsupported. Under conditions of moderate winds this unsupported area bends with respect to the lens. The result is that the amount of energy reflected back to the feed element 12 changes thereby adversely affecting the match and isolation at the feed.

The serious disadvantages set forth above are overcome in the arrangement of the present invention by locating the supports 15 in the concentric shadow or null areas 16, 17 of the lens radiation pattern. These null or shadow rings are caused by the broad banding step cutbacks on the edges of the lens slats facing the feed element. As many supports 15 may be employed as required according to the maximum wind loads anticipated.

In the embodiment of Figs. 1 and 2 the lens as well as the shield reflect back to the feed a portion of the energy impinging thereon. It has been discovered that spacing the shield /2 wavelength at the operating frequency from the front or flat surface 30 of the lens, as shown in Fig. 2, causes the two reflections to cancel resulting in minimum overall reflection back from the lens-shield combination.

Figs. 3 and 4 show in greater detail the structure of support 15. The support consists of a threaded portion 18,.a pair of flanges 19 with apertures therethrough located in one end thereof and an annular portion 20 intermediate the threaded and flange portions thereof. The lens slats 11 are secured to the flange portions 19 by means of rivets 21 or screws or bolts. The support passes through an aperture in lens shield 13. Neoprene washers 22 located adjacent the lens shield are for waterproofing purposes. The lens shield is securely held in place on the support by means of nut 23 and lock. Washer 24.

If desired, rather than being spaced from the lens shield, the lens may be flush mounted thereon. This may readily be accomplished by forming cut-out portions 25 in the lens slats so that the slats may be fitted over annular portion 20 of mounting member 15 and neoprene washer 22, as indicated in Fig. 5.

What is claimed is:

1. In an arrangement including a lens for focusing radio waves into a beam pattern having at least one null portion adjacent said lens and a shield substantially transparent to radio waves adapted to be mounted to said lens in the path of said beam pattern: mounting means located in said null portion of said beam pattern for securing said shield to said lens, whereby said mounting means has minimized effect on said beam pattern.

2. In an arrangement including a lens for focusing radio waves into a beam pattern having null portions adjacent said lens and a shield substantially transparent to radio Waves adapted to be mounted to said lens in the path of said beam pattern: mounting means located in said null portions of said beam pattern for securing said shield to said lens, whereby said mounting means have no elfect on said beam pattern.

3. A lens antenna comprising, in combination, a metallic lens for focusing radio Waves into a beam pattern having null portions adjacent said lens; a shield substantially transparent to radio waves adapted to be mounted to said lens in the path of said beam pattern; and mounting means located in said null portions of said beam pattern for securing said shield to said lens, whereby said mounting means have no effect on said beam pattern.

4. A lens antenna as set forth in claim 3 wherein said lens includes a plurality of lens slats, said lens slats being formed with cut-out portions along a portion of an edge thereof adjacent said mounting means to permit flush mounting of said shield to said lens.

5. A lens antenna comprising, in combination, a metallic lens including a plurality of lens slats formed with projecting portions along one edge thereof so as to provide a circularly stepped lens; a feed element located on the focal line of said lens and on the side thereof closest the circular steps thereon for illuminating said lens, whereby the beam pattern of said lens includes annular null portions; a lens shield substantially transparent to radio waves adapted to be mounted to said lens in the path of said beam pattern; and a plurality of mounting elements located in said null portions of said beam pattern securing said shield to said lens at a plurality of difierent points, whereby said mounting elements have no effect on said beam pattern.

6. In an arrangement including a lens for focusing radio waves into a beam pattern having a null portion near the center of said lens and other null portions spaced further from the center of said lens, and a shield which reflects a minor portion of said radio waves adapted to be mounted to said lens in the path of said beam pattern: mounting means located in said null portions of said beam pattern for securing said shield to said lens, whereby said lens shield is prevented from being displaced toward and away from said lens thereby preventing changes in the amount of energy reflected by said lens shield to a feed element, and said mounting means have no effect on said beam pattern.

7. .A lens antenna comprising, in combination, a metallic lens for focusing radio waves into a beam pattern; a feed element located substantially at the focus of said lens; and a shield substantially transparent to radio waves mounted to the side of said lens opposite said feed element, said shield being spaced substantially one half wavelength from said lens at the operating frequency of said lens antenna.

8. A lens antenna as set forth in claim 7, wherein said lens is a stepped lens providing a beam pattern having null portions adjacent said lens, and further including a plurality of supports securing said shield to said lens, said supports being located in said null portions of said beam pattern.

No references cited. 

